Vertical-cavity surface-emitting lasers (VCSELs) represent a new generation of high-speed, high-efficiency devices ideal for high-bandwidth optical sensors and data communication in military applications.
VCSEL technology offers many advantageous properties, such as micro-scaled device size, two-dimensional scalability, and lower power consumption. As an optical transmitter, a VCSEL emits a beam having a Gaussian profile with an extremely large beam divergence (half angle of more than 15 degrees). This property makes the direct integration of VCSELs to the receiving detectors or optical fibers very difficult since there is a large loss in device coupling. For instance, a single-mode fiber has a core diameter of only a few microns (μm) with a receiving half-angle of 5 degrees. Large multi-mode fibers require a receiving half-angle less than 10 degrees. For a two-dimensional system, a precise alignment that is necessary for the integration would become an even bigger challenge.
Presently, external optical components are used to control the laser beam in a VCSEL transmitter and interconnect system. However, it has been found that excessive optical parts could cause significant optical aberrations and insertion losses and increase substantially the packaging costs. It is extremely important to minimize the losses for a VCSEL interconnect system since the total optical power involved is very low.
There have been research activities in recent years on beam shaping of vertical-cavity lasers using microlenses. The structures are basically in two categories: (1) 980 nm substrate-emitting lasers with microlenses monolithically integrated on the GaAs substrate; and (2) Vertical-cavity lasers that are monolithically or externally integrated with diffractive optical elements (DOE) for output beam control.
As an example of the former, U.S. Pat. No. 5,073,041 entitled “Integrated Assembly Comprising Vertical Cavity Surface-Emitting Laser Array with Fresnel Microlenses” discloses beam shaping for a 980 nm VCSEL with a Fresnel microlens array on a GaAs substrate. The laser beam transmits through its GaAs substrate which has to be within a certain thickness in order to become transparent to the 980 nm beam. The required monolithic fabrication process is also very complicated with little flexibility.
Another example that is related to the background of the current invention, U.S. Pat. No. 6,583,445 entitled “Integrated Electronic-Optoelectronic Devices and Method of Making the Same”, discloses an integrated electronic-optoelectric module comprising of VCSELs and photodetectors with ultrathin silicon-on-sapphire electronic circuitry composite substrate. Such modules will be useful for electro-optical interconnects for computing and switching systems.
In sum, the optical outputs of conventional optical transmitters exhibit a considerably large beam divergence. It is very difficult to integrate optical transmitters to the receiving detectors and optical fibers. External optical components can be employed to shape the beams, but they will cause significant optical aberrations and insertion losses, increase packaging costs, and ultimately reduce the power efficiency. Thus, an outstanding need remains for an improved technique to shape the output beam of a VCSEL to achieve specific optical functionalities. Such an approach should lead to optically controlled, low-loss, high-efficiency, and high bandwidth transmitters and interconnects.